Crivello, U.S. Pat. No. 4,173,551, assigned to the assignee herein, discloses heat curable compositions of epoxidic prepolymers and aromatic onium salts such as diaryliononium salts in combination with various cocatalysts such as copper salts. Such compositions are useful to make, for example, protective, decorative and insulating coatings, potting compounds, printing inks, sealants, adhesives, molding compounds, wire insulation, textile coatings, laminates, impregnated tapes, varnishes, and the like. Such compositions are so highly active, however, that they tend to cure at room temperatures within a few days' time and this makes shelf storage somewhat of a problem. In applicant's copending United States patent application, Ser. No. 53,259, filed June 29, 1979, mineral filled such compositions are shown to be especially useful in filling, bridging or bonding substrates such as are encountered in the manufacture and repair of automobiles, and the like. Here, again, however, shelf-storage life is rather shorter than would be desired. The problem has been somewhat alleviated in the past by using a special form of the copper salt co-catalyst, namely, copper stearate, but this is not too satisfactory because such a salt must be milled into the epoxy in a separate operation.
In the said copending application, Examples 12 and 13 show the addition of, respectively, about 25 parts by weight per 100 parts of prepolymer of a carboxyl terminated copolymer of butadiene and acrylonitrile and of a hydroxy terminated copolymer of butadiene and acrylonitrile to a mineral-filled composition comprising epoxy resin prepolymer, a complex halonium salt and a copper salt, for the stated purpose of enhancing the flexibility of the cured composition. The disclosures of the patent and of the application are incorporated herein by reference.
It has now been discovered that the copolymers of butadiene and acrylonitrile mentioned-above, even in surprisingly small quantities, serve to stabilize such compositions against cure at normal temperature, but yet to permit rapid cure in the normal fashion at elevated temperatures. For example, a epoxidic ester prepolymer when catalyzed with as little as 1 percent of diphenyliodonium hexafluoroarsenate and 0.04 percent of copper naphthenate has a room temperature shelf life of only 2-3 days. The same composition, but with a small amount, e.g., 20 percent of a carboxyl- or hydroxy-terminated butadiene and acrylonitrile copolymer, has a room temperature shelf life of greater than twelve weeks. Even so, at 150.degree. C., gel time is only 50 seconds, demonstrating the latent nature of the catalyst composition.